The intestinal epithelium has a remarkable capacity to repair and regenerate after injury; however, our understanding of underlying mechanisms is incomplete. We recently reported that following intestinal epithelial injury by the chemotherapeutic, doxorubicin, intestinal stem cell (ISC) numbers increase during crypt regeneration. Importantly, concomitant with ISC expansion, we demonstrated dramatic expansion of the Paneth cell compartment associated with increases in both Paneth cell number and size. Paneth cells reside at the base of small intestinal crypts, and intercalated among them are ISC. This places Paneth cells in an advantageous position to influence ISCs, and impact ISC survival, proliferation or cell fate decisions. The mechanisms that drive expansion and function of Paneth cells, particularly during epithelial repair are not well understood. Our centra hypothesis is that PC expansion is regulated by bacterial and Wnt signaling and is critical in providing a microenvironment favorable for ISC expansion and epithelial repair following hemotherapy-induced intestinal mucosal injury. To test this hypothesis we propose the following three specific aims: 1) Define how bacterial signaling in intestinal epithelium or PC affects PC expansion following Dox- induced mucosal injury, 2) Define the role of Wnt signaling in expansion of the PC compartment during repair, and 3) Define the functional significance of PCs during epithelial repair after chemotherapy-induced injury. The objective of this proposal is to generate a better understanding of mechanisms involved in PC expansion and the function of PCs, particularly following mucosal injury. The significance of these studies will be an understanding of the mechanisms that control PC expansion following mucosal injury by a commonly used chemotherapeutic agent that impairs intestinal function as well as the function of PC in epithelial repair. These studies will also define the enteric bacterial population before and after intestinal damage. In addition, we will elucidate PC transcriptome and secretome from untreated and damaged intestine. Innovations include novel mouse models to interrogate bacterial and Wnt signaling roles in chemotherapy-induced injury, PC function and PC expansion. Delineation of new potential avenues of therapeutic intervention to protect and induce PC expansion after chemotherapy-induced injury of intestinal epithelium could have significant clinical impact and relevance to other situations of intestinal injury. Currently, we know very little about what drives ontogeny of Paneth cells following injury, but understanding such events would provide potential avenues of therapeutic intervention either for protecting Paneth cells or for inducing expansion.